In order to print an image such as a text document, a graphic or a photograph, on a print medium such as paper or transparency material, a typical high quality color inkjet printer prints a band, or “swath”, at a time of colored ink drops which correspond to the data pixels that comprise the image. Typically, four different color inks (cyan, magenta, yellow, and black) are used by the printer to print the range of colors contained in the image. By printing successive swaths, the document or image is completely formed on the print medium. Such inkjet printers are described by W. J. Lloyd and H. T. Taub in “Ink Jet Devices,” Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988). The basics of this technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No.5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994)], incorporated herein by reference.
When a swath contains one or more relatively large regions which are to be printed in black, such as the interior portions of textual characters, it is important to achieve a uniform, high optical density, or darkness, in the black regions. One technique that is commonly used to produce black regions with uniform high optical density is under/overprinting. See, e.g., U.S. Pat. No. 6,132,021 to Smith et al., assigned to the assignee of the present invention and incorporated herein by reference in its entirety. In addition to printing these regions with black color ink (known as “process black” or “true black”), these regions may also receive drops of cyan, magenta, or yellow inks which are deposited underneath (underprinting) or on top of (overprinting) the drops of black ink. Particularly when the types of ink are different (for example, the black ink is typically pigmented, while the cyan, magenta, and yellow inks are generally dye-based), the under/overprinting results in improved optical density because the underprinted inks tend to pre-treat the surface of the print medium, and because the cyan, magenta, and (if used) yellow ink drops combine on the print medium to produce a “composite black” coloration. The amount of under/overprinting must be carefully controlled, however, in conjunction with the level of pixel depletion so as to avoid oversaturating the print medium with too much ink that will not dry quickly enough to avoid blotting onto the preceding or following page in the output tray of the printer, or smearing when handled by the user.
Unfortunately, the printhead containing the individual printing elements which controllably deposit the black ink drops on the print medium may degrade during the course of its useful life, resulting either in misdirected ink drops which are not deposited in the intended location, or no ink drops at all. Each such defective printing element in the printhead will typically produce a row or line of unprinted space on the print medium. Even if typical amounts of cyan, magenta, and yellow inks are deposited in these unprinted spaces via under/overprinting, the absence of true black ink will create areas of diminished optical density. Accordingly, it would be highly desirable to have a way to mitigate the adverse impact on the uniformity and optical density of black printed regions due to defective printing elements in the black printhead of the printer.